The only memory most will have of the Dolphins’ third exhibition game was Dustin Keller’s gruesome knee injury.

But before Keller was carted off the Reliant Stadium field Saturday night, two other key free agent pickups — Mike Wallace and Brent Grimes — showed glimpses of why the Dolphins opened up their wallets in the offseason.

Wallace had three catches for 58 yards — including a 9-yard touchdown grab — and Grimes made a diving interception to snuff out the Texans’ first drive.

The Texans ultimately beat Miami 24-17 in a game largely decided by backups.

For Grimes, the play was a payoff worthy of the work he has put in during training camp. Grimes, who has less than a year removed from a torn Achilles tendon, has been far and away the team’s best corner in practice.

Said Grimes of the interception, “I just read the route and broke. ... It’s always a positive when you can get interceptions, turnovers.”

Wallace’s emergence, however, was even more notable. He had missed the first preseason game with a groin injury and didn’t have a catch in his Dolphins debut a week ago.

Earlier in the week, he and Ryan Tannehill were forced to answer questions why. They silenced those doubts with one beautiful throw.

On the Dolphins’ third series, Wallace got behind Johnathan Joseph on a deep sideline route and pulled in a rainbow pass from Tannehill. The completion was good for 33 yards.

Three plays later, they hooked up again — this time for a touchdown. Wallace shook free of coverage and reeled in a laser throw from Tannehill.

Yet when asked by CBS-4 at halftime about Wallace’s production, Dolphins coach Joe Philbin praised an underappreciated part of their $60 million man’s game.

“He had on the touchdown run down here in the end zone, he had an outstanding block, which is something that a receiver has to do as well,” Philbin said.

By touchdown run, Philbin was referring to a 4-yard dash by Lamar Miller. But that score was far less celebrated; it came on the drive Keller got hurt.

Early in Miami’s fifth possession, Keller went down after taking D.J. Swearinger’s helmet to his right knee.

The severity of the injury was not immediately known, but ESPN reported midway through the second half that Keller was set to undergo an MRI late Saturday night.

“You all saw Dustin get carted off the field,” Philbin said. “You hate to see any player in that situation. We’re all wishing Dustin well, and we’re hoping his return is sooner rather than later.”

If lost for the season, it would be a tough blow for Tannehill, who had his best game of the preseason. Tannehill completed 10 of 15 passes for 141 yards and the touchdown, good for a quarterback rating of 119. He had targeted Keller twice, completing one pass for 14 yards.

Meanwhile, Daniel Thomas continued his preseason ascent, rushing for 25 yards on four carries. He appears to be gaining on Miller for the starting running back job. Miller had just 10 yards on five carries.

“He is running the ball hard,” Philbin said of Thomas.

The first-string offense certainly finished better than it started. Josh Samuda surrendered a sack to Antonio Smith on the second play from scrimmage, leading to a three-and-out. Back-to-back penalties doomed the next possession before it even started.

As for the defense? It had no trouble getting revved up. After Grimes’ pick — he leapt in front of DeAndre Hopkins for the interception — the Dolphins forced a punt on Houston’s second possession.

Cameron Wake got into the act with a drive-ending sack. And the starting defense’s night was done before the first quarter ended.

But the first-string offense, which had just one score on its first nine drives of the preseason, needed a wake-up call.